1. Field of the Invention
This invention deals generally with diagnostic methods and apparatus for determining the condition of bones. In particular, it discloses a method for determining non-invasively, in vivo, the condition of certain bones in humans and animals. The invention is distinguished in part by the emission into the patient, adjacent a bony member, of a wideband ultrasound signal, and by the use of a multivariable analysis of the emerging ultrasound signal to analyze the architecture of scattering medium i.e., the bony member. The multivariable analysis may include a statistical study such as a multivariable discriminant analysis.
2. Discussion of the Problem and Objects of the Invention
This invention pertains to detecting bone disease and other abnormal gone conditions. The invention is intended specifically for evaluation and diagnosis of humans; but with obvious modification of the illustrative embodiments, the invention can be used to evaluate animal bones, also. Therefore, the term "patient" is used herein to include both human and animal subjects. The main object is to diagnose the presence of osteoporosis. However, other conditions may also be detected using this method, including bone condition variations following renal failure and periodontal disease due to bone deterioration. The invention may also be useful both for detecting fractures and for assessing, quantitatively, the healing of fractures.
The human skeleton is composed of tubular (long and hard) and cancellous (spongy) bones, each of which is composed of specific proportions of cortex (compact bone) and trabecullae (connective strands). Tubular bones, which are composed largely of cortex, dominate the appendicular skeleton that make up the limbs; cancellous bones, which are composed primarily of trabecular bone, dominate the axial skeleton of the vertebral column and pelvis.
In both cortical and trabecular bone, the collagen fibrils extend throughout. The difference between the two is actually one of degree, depending upon the form the network of collagen fibrils takes. In general, the fibrils may be separated such that the network is a network of rods. They may also be more closely spread, so that the network appears as a network of plates connected by rods. In both cases, a certain volume of the bone will comprise mineralized collagen fibrils, and a certain volume will comprise non-osseus material known as "marrow."
Whether a particular bone assumes a trabecular or cortical formation is thought to depend largely upon the function that bone serves. The method of bone formation is not well understood; however, it is believed that the process of bone accretion responds in some manner to stresses experienced by the bone. Therefore, that region of a bone that experiences relatively high stresses, such as the diaphysis of the tibia of the leg, tends more toward cortical bone. Regions of a bone that experience low stress tend more to be trabecular. In most sites of trabecular bone, the trabecular mass is surrounded by a relatively thin layer of cortical bone. The patella is mostly trabecular with a subcortical layer just beneath the anterior surface.
As used herein, "patella" refers to the thick, flat triangular bone that forms the anterior part of the knee; "kneecap" refers to the patella and surrounding soft tissue; and "tibia" refers to the anterior bone of the lower leg.
The principal target of the invention (that is, the primary condition for which diagnosis is sought), osteoporosis, is a disease of unknown cause which afflicts people, generally, as they age. Osteoporosis afflicts women more often than men; and of women, more often after menopause. White women are more often stricken than women of other races. Osteoporosis is manifest as an absolute decrease in bone tissue mass. The bone that remains is, however, normal. A person suffering from osteoporosis loses a greater proportional amount of trabecular bone than cortical bone.
Another target of the invention, periodontal disease, involves loss of bone in the mandible and maxilla, with consequential loosening of teeth. Heretofore, the progress of bone loss in the jaw has been monitored with X-rays, which can only reveal the presence of periodontal disease after substantial bone loss has already occurred.
Consequently, it is highly desirable to provide a means for detecting bone condition (and, hence, changes in bone condition--including, but not limited to the loss of bone material and attendant decrease in bone density and elasticity). Optimally, such means is non-invasive, accurate, sensitive, easy to use and can be made generally available. This is not, however, true of the prior art techniques, in general.
In recent years, several methods have been proposed for the early diagnosis of osteoporosis. These methods include Neutron Activation Analysis of Total Body Calcium (TBC), single photon aborptiometry (of wrist and os calcis) and dual photon aborptiometry (of the spine and neck of the femur), Computer Aided Topography (CAT scanning) and methods of ultrasound analysis. The advantages and disadvantages of these techniques are well documented in the literature and are summarized in commonly assigned U.S. patent application Ser. No. 06/870,175, filed June 3, 1986, titled "Ultrasound Method and Apparatus for Evaluating, In Vivo, Bone Conditions," which is incorporated by reference herein in order to avoid needless repetition.
For in vivo, non-invasive analysis of bones in patients, measurement by use of ultrasound already has been performed to some extent. Ultrasound measurement, however, is complicated, in part, by the presence of soft tissue surrounding most bones. The propagation speed of an ultrasound signal injected transdermally is affected by passage through the soft tissue surrounding the bones, as well as by passage through the bone(s) of interest. It is possible to minimize the effect of the soft tissue. Initial attempts to minimize the effect of the soft tissue were encumbered by requirements for rather sophisticated echo analysis and careful measurement. See, for example, U.S. Pat. No. 4,361,154, "Method for Establishing, In Vivo, Bone Strength." A more advanced system is presented in U.S. patent application Ser. No. 06/870,175, filed June 3, 1986 in the names of George W. Pratt, Jr. and Paul Duchnowski and titled "Ultrasound Method and Apparatus for Evaluating, In Vivo, Bone Conditions." That system employs an ultrasonic pulse having at least two components of distinguishable waveshape or frequency content in a range from about 100 kHz to about 3 MHz, launched transdermally through a bony member such as the patella. A variety of techniques are used to distinguish between the signal transmitted by the soft tissue and the signal transmitted by the bone, to facilitate assessment of bone condition. These techniques include comparing the transit times through the bony member of energy in a first frequency range and energy in a second frequency range; evaluating the transfer function through the bony member of the portion of the signal travelling through the bony member; evaluating a gain function of the power spectrum of the portion of the signal transmitted through the bone, including evaluation of the area under such gain function and/or the magnitude and location of its peak amplitude. The gain function whose area was evaluated was the absolute value squared of the system function, the latter being defined as the received signal normalized by the transmitted signal--i.e., the transfer function of the channel existing between the two transducer faces. Such prior application also teaches the deduction of velocity of ultrasound energy through the bony member by determining the duration of travel of the ultrasound signal through the bony member and soft tissue, and adjusting such composite velocity by a soft tissue normalization factor. This information is then compared to a database of prior measurements for the same patient and/or for a selected population (which may be, for example, the population at large or a selected population of like age, sex, race, etc.), to determine a probability that the patient s bone condition is abnormal.
The techniques of that previous application represent a significant improvement over the prior art, but they still have left room for further improvement. The measurements based on signal transit time and velocity are sensitive to transducer orientation and placement, for example. The gain function evaluations show promise but do not provide a basis for an individual diagnosis having the degree of confidence physicians demand.
The principal object of this invention, therefore, is to provide a superior method and apparatus of ultrasound diagnosis of bone condition.
This broad object may be formulated in several other ways and in subsidiary and related objects: (1) to evaluate bone condition in patients using a non-invasive, in vivo technique; (2) to evaluate bone condition without subjecting patients to substantial doses of ionizing radiation; (3) to evaluate bone condition in patients economically and quickly; (4) to provide a method or methods of evaluating bone condition that may be performed safely and economically many times over the course of several years; and (5) to provide a method or methods of evaluating bone condition that may be performed by a technician without the need for expensive equipment or especially careful transducer placement or measurement.